Typically, it is necessary to control process control fluids in industrial processes, such as oil and gas pipeline distribution systems and chemical processing plants. In some industrial processes, butterfly valves are used to control the flow of process fluid. Generally, the industrial process conditions, such as pressure conditions, operational temperatures, and the process fluids dictate the type of valve components, including the types of butterfly valve seals that may be used.
A portion of a known butterfly valve 50 is shown in FIG. 1. The butterfly valve 50, which may be, for example, the 8510 valve made by Fisher®, a division of Emerson Process Management of St. Louis, Mo., uses a polytetrafluoroethylene (PTFE) seal. In a typical PTFE seal, a PTFE seal ring 52 is secured in a valve body 54. The PTFE seal ring 52 makes contact with a disc 56 when the valve 50 is closed to form a seal therebetween. PTFE seals, such as that depicted in FIG. 1, provide excellent sealing performance compared to metal seals and provide a relatively long seal life. PTFE seals also provide a reduction in the amount of torque needed to unseat a disc (e.g., the disc 56) from the seal (e.g., the seal ring 52), but are limited to process applications that expose the seal to temperatures below 450 degrees Fahrenheit.
Graphite laminated seals, such as a seal 62 used in a butterfly valve 60 of FIG. 2 are also known. The graphite laminated seal 62 of FIG. 2 is generally used in butterfly valves known as triple offset valves. Compared to conventional double offset valves, triple offset valves typically have a larger offset between the center of rotation of the valve shaft (not shown) and the center of rotation of a disc 64. The offset causes the disc 64 and the seal 62 to travel along an eccentric path as the disc 64 moves into and away from a seat 66, thereby substantially reducing the contact region of the expanded graphite laminate seal 62 and the seat 66 during closure. As further distinguished from a double offset valve, the cross-section of the disc 64 of the triple offset valve 60 is typically elliptical rather than circular to further reduce contact area between the seal 62 and the seat 66 near closure. As is known, the triple offset valve 60 is configured to reduce wear in any applications (e.g., throttling or on-off) by reducing the contact or engagement area between the seal 62 and the seat 66 when the disc 64 and the seal 62 are unseated (i.e., operating near the seat 66 when opening or closing).
Generally, the seal 62 is rigidly attached to the disc 64 and the seat 66 is integral to the valve body 68. Triple offset designs such as that shown in FIG. 2 can be disadvantageous due to the high torque required to drive the disc 64 and the seal 62 into and away from the seat 66 to ensure tight shutoff. Additionally, this type of seal is difficult to maintain. For example, if there is any damage to the seat 66, which is integral to the valve body 68, the valve body 68 also requires repair or replacement.
Metal seals have also traditionally been used in butterfly valves. One such metal seal, which is shown in the portion of a valve 70 shown in FIG. 3, is the metal seal used in the 8510B valve also made by Fisher®, a division of Emerson Process Management of St. Louis, Mo. In the seal shown in FIG. 3, a cantilevered metal seal ring 72 contacts a disc 74 to form a seal therebetween. Metal seals are well suited for use with high temperature and high pressure process applications, but generally are more susceptible to wear and, thus, require greater maintenance and incur greater cost.
There have been numerous attempts to combine the characteristics of at least two of the known seal types previously described. One such attempt is shown in FIG. 4, which illustrates a portion of a valve 80 with the fire safe seal by Xomox® of Cincinnati, Ohio. The fire safe seal illustrated in FIG. 4 combines elements of a PTFE seal and a metal seal. As depicted in FIG. 4, a primary PTFE seal 82 is retained within a receiving channel 84 of a secondary metal seal 86. The fire safe seal is retained within a valve body 88 by a seal ring retainer 90 and is configured so that upon retention within the valve body 88 a preload of the fire safe seal results in a bend or flexure 92 in the metal seal 86 similar to that of a belleville washer. This preload creates a spring force so that when a disc 94 contacts the seal, the spring force drives the fire safe seal into contact with the disc 94 and a fluid seal is formed between the PTFE seal component 82 and the disc 94. In operation, the primary PTFE seal component 82 is sacrificial. For example, in the case of a fire where temperatures surrounding the PTFE seal component 82 exceed 450 degrees Fahrenheit, the PTFE component 82 may be consumed (i.e., sublimated or burned), but the spring force provided via the flexure 92 causes the metal seal 86 to contact the disc 94 to maintain the fluid seal therebetween. However, the type of fire safe seal depicted in FIG. 4 is susceptible to fatigue failures at the flexure 92.